Tach is fed from igniter, perhaps check connections at igniter, along with verifying it is grounded well. If you search, there is a thread that shows how to add additional grounding to igniter to insure it is grounded well... though the benefits of grounded mod, reach much farther then insuring it is not causing your tach not to work though... otherwise the power side also feeds other gauges so if that is the only not working, than imo it is either not getting signal from igniter or something has gone wrong on the printed circuit fabric on the back of the combination reader assembly...
Good luck, would not want to live without my tach...